Class-G amplifier configuration can exhibit distinct advantages over conventional class-AB and class-D topologies. Class-G amplification, by employing multiple supply rails of differing voltages in the output stage and selectively commutating among the supplies as the output voltage varies, is able to effectively minimize the voltage drop across the output power devices at any given instant, thereby minimizing power dissipation in the amplifier and reducing overall power consumption. Class-AB, with two fixed rails, is inherently less efficient because no attempt is made to minimize VI losses in the output devices. Class-D employs switched output devices in which the VI product is, on average, very small and thus is generally very efficient, but requires extra support components in the form of low pass filtering at its outputs. Class-D topologies have the additional disadvantages of producing electromagnetic interference (EMI) emissions and exhibiting output voltage overshoots that can be difficult to manage and which can compromise reliability.
A Class-G implementation may employ any of a number of available topologies to minimize the VI product, and therefore power dissipation, in the output stage. FIGS. 1 and 2 illustrate generally examples of a bridge-tied load (BTL) amplifier configuration wherein two class-G amplifiers—effectively two class-AB amplifiers with selectable supply rails—drive identical signals of opposite phase into a load. FIG. 3 illustrates generally an example of a rail selector. FIGS. 4 and 5 illustrate generally associated waveforms for the example amplifier configurations of FIGS. 1 and 2.
Another example of a BTL amplifier configuration includes an ST Microelectronics TDA7563 car audio amplifier chip. In this example, at any given time, only one of the two amplifiers is on, and the other is off. The “on” amplifier drives the full signal at one end of the load, and the “off” amplifier is simply switched to GND. As the output signal changes polarity, the two amplifiers alternate states. This cuts the power dissipation roughly in half versus a conventional Class AB configuration.
In other examples, a common mode voltage of a pair of bridged amplifier outputs can be selectively shifted as needed to lie between any pair of a multiplicity of varyingly spaced power rails, with the particular pair of rails chosen at any given instant possessing the minimum spacing needed to accommodate the amplitude of the output signal being issued at that instant, and with the amplifier's output devices selectively conducting current out of the higher of the two chosen rails of the selected pair, through the load, and into the lower of the two chosen rails of the selected pair.